Other occasions in Scripture have been given where humans did not believe a prophecy regarding the coming of a child. Remember what Elisha said to the Shunammite woman ‘At this season, when the time cometh round, thou shalt embrace a son.’ And she said to him: ‘ Nay, my lord, thou man of God, do not lie unto thy handmaid (II Kings 4: 16). She too did not believe Elisha God’s prophet when he made this prophecy. But at the appointed time the child did arrive (II Kings 4: 17).
The promised son was born when Abraham was a hundred years old and Sarah was ninety. Her womb and ovaries had been restored to give birth as well as her breasts to nurse. Sarah’s name means a princess, which signifies that many nobles would come from her. The Orthodox Jews see the word “children” as apposed to child as significant (Genesis 21: 7). They reason that she would nurse many children to prove the birth to the other princesses that came to see the boy believing rather that Sarah had brought home an abandoned child from the marketplace. The Bereshit Rabbah honoring Sarah expands on this concept.

Our mother Sarah was extremely modest. Said Abraham to her: ‘This is not a time for modesty, but uncover your breasts so that all may know that the Holy One, blessed be He, has begun to perform miracles.’ She uncovered her breasts and the milk gushed forth as from two fountains, and noble ladies came and had their children suckled by her, saying, ‘We do not merit that our children should be suckled with the milk of that righteous woman.’ The Rabbis said: Whoever came for the sake of heaven became God-fearing (Bereshit Rabbah 50: 9).